Collation shrink concerns the bundling of items together using heat shrinkable film. Collation shrink is used for a very wide variety of applications and notably for the secondary packaging of food or drinks. Examples include metal cans and plastic bottles.
Typically, films are applied at room temperature and placed under a heat source to shrink. Suitable performance characteristics on the shrink packaging line include sufficient stiffness allowing the film to be correctly wrapped around the items being packaged, sufficient dimensional shrinkage to ensure a snug fit, and a low enough Coefficient of Friction (COF) for machinability and package handling. Films appropriate for use as collation shrink must have a high thermal shrink force to ensure a tight fit and high tensile strength to withstand handling and abuse during transportation.
In addition, the packaging must have excellent display properties including gloss (preferably under different angles to maximize appeal), haze (or “contact clarity”) and clarity (“see-through clarity”).
Finally, the collation shrink film manufacturer wants the properties of a low melt pressure, and the ability to use low motor power, both allowing higher production rates.
While it is known how to improve many of the above properties individually, currently available structures do not combine all of the properties satisfactorily in a film having sufficiently thin gauge to be commercially attractive.
Polyethylene is an attractive component to use in collation shrink film. Various types of polyethylenes are known in the art. Low density polyethylene (“LDPE”) can be prepared at high pressure using free radical initiators and typically has a density in the range of 0.916-0.940 g/cm3. LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene in the same density range, i.e., 0.916 to 0.940 g/cm3, which is linear and does not contain large quantities of long chain branching is also known; this “linear low density polyethylene” (“LLDPE”) can be produced with conventional Ziegler-Natta catalysts or with single site catalysts—such as metallocene catalysts. Relatively higher density LDPE or LLDPE, typically in the range of 0.928 to 0.940 g/cm3 are sometimes referred to as medium density polyethylene (“MDPE”) or Linear Medium Density Polyethylene (LMDPE). Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), i.e., polyethylenes having densities greater than 0.940 g/cm3, and are generally prepared with Ziegler-Natta catalysts, chrome catalysts or even single site catalysts such as metallocene catalysts. Very low density polyethylene (“VLDPE”) is also known. VLDPEs can be produced by a number of different processes yielding polymers with different properties, but can be generally described as polyethylenes having a density less than 0.916 g/cm3, typically 0.890 to 0.915 g/cm3 or 0.900 to 0.915 g/cm3.
U.S. Pat. No. 6,187,397 teaches a 3-layer co-extruded heat-shrinkable film devoid of metallocene polyethylene. The patent teaches that prior art “high clarity” heat-shrinkable polyethylene films are obtained by coextrusion of three layers comprising a central layer of predominantly (>50 wt. %) free-radical polyethylene having a relative density of 0.918-0.930, optionally with HDPE to confer stiffness, sandwiched between two layers of predominantly (80-90 wt. %) metallocene linear polyethylene having a density of 0.918-0.927.
U.S. Pat. No. 6,340,532 discloses shrink films manufactured from “pseudohomogeneous” linear low density polyethylene resins preferably prepared with an advanced Ziegler Natta catalyst. Various deficiencies of “homogeneous” resins, i.e., metallocene resins, used in prior art shrink films are discussed.
U.S. Pat. No. 6,368,545 teaches a high clarity multilayer blown coextruded film prepared using special methods, wherein a film is described having a central core of HDPE.
U.S. Pat. Application No. 20020187360 is directed to a heat shrinkable, co-extruded polyethylene film laminate having a relatively low melting point core layer comprising a linear low density polyethylene (LLDPE) having a density of 0.910-0.930 g/cm3 and a linear very low density polyethylene (VLDPE) having a density of 0.880-0.915 g/cm3, sandwiched between two relatively higher melting point surface layers comprising a linear low density polyethylene and a linear high density polyethylene.
WO 01/44365 describes a homogeneous blend of a metallocene-catalyzed medium density polyethylene (mMDPE) with a low density polyethylene (LDPE) to produce blown films. The blend may be coextruded between layers of LDPE to make blown films taught in the reference as having the good optical properties of LDPE and the good mechanical and processing properties of MDPE.
Additional patents of interest include U.S. Pat. No. 6,492,010, U.S. Statutory Invention Registration H2073, WO 95/00333 and EP 0597502.
The high gloss provided by metallocene polyethylenes is a very attractive property. However, film layers comprising metallocene polyethylene have a very high coefficient of friction in the absence of specific additives. These additives, in turn, detract from the optical properties desired in collation shrink films. A film exploiting the high gloss capabilities of metallocene polyethylene that can be produced efficiently and having the properties sought in a collation shrink film is highly desirable.
The present inventor has surprisingly discovered that an improved collation shrink film may be achieved by a structure having a core layer comprising HDPE and skin layers comprising metallocene polyethylene and optionally at least one of HDPE or LDPE resins.